1. Field of the Invention
The present invention is related to the processing of multi-colour images for reproduction on a printing or copying system. Particularly of interest are systems for forming images composed of a plurality of colour separation images on an image-receiving member wherein the marking particles of the respective colours associated with the respective colour separation images are positioned in superimposed relationship.
2. Discussion of the Related Art
Marking particles of a limited number of colours, being the process colours, are available on each multi-colour printing system to render a colour image. In digital colour printing, continuous tones are rendered by halftoning the separation images in the process colours. The process colours are a limited number of colours of marking particles available on each multi-colour printing system to render a colour image. Usually a distinction can be made between these colour printing systems based on the kind of marking particles used, e.g. ink or toner, the imaging process employed, e.g. magnetography, or electro(photo)graphy, or inkjet, the productivity or the media range. A distinction can, however, also be made dependent on how the multi-colour image of marking particles is composed. In the majority of commercially available digital multi-colour printing systems, the multi-colour image of marking particles is composed of a plurality of registered colour separation images of marking particles, where the image dots of marking particles of the respective process colours associated with the respective colour separation images are superimposed and as such form a layered structure of marking particles. The marking particles pile height depends on the marking particle size, the halftoning and the number of process colours available, and varies with image density.
In a first halftoning approach, the colour separation image of each process colour is halftoned using a different screen for each separation image. The amount of overlap between image dots of different process colours depends on the image density and the pixel filling in sequence associated with the respective screens. A disadvantage of this approach employing a plurality of different screens is its sensitivity for creating Moiré patterns. Moiré patterns are visible distortions in a rendered multi-colour image caused by interference patterns generated by combining halftone screens. Although it is known that the visible effect of Moiré patterns can be reduced by angling the halftone screens using predetermined screen angles, avoiding Moiré becomes particularly troublesome in colour printers where four or more process colours can be rendered. Therefore using a different screen for each process colour is not an option when more than four process colours are available on the colour printer to render a multi-colour image.
In a second halftoning approach, the same screen is used for each process colour. This approach yields maximum overlap between image dots of the respective colours and by consequence minimal area coverage. Particularly at low image densities, the images rendered according to this second approach are highly sensitive to graininess. Graininess is a perceived feature of a rendered colour which is related to how uniformly the coloured marking particles have been developed on the image receiving member.
The above-mentioned approaches have some further inherent disadvantages. Firstly, because the marking particles of the different process colours are superimposed, the total marking particles pile height can be high, particularly in full colour high density image parts. Particularly in case the marking particles are toner particles, as the size of toner particles is typically in the micrometer range, this may limit the amount of process colours which may be used to render an image as an increased number of process colours also may increase the maximum marking particles pile height. Besides the fact that a high total marking particles pile height is noticeable to the customer both visually and palpably, this may also negatively influence medium curl and transport as well as reduce the resistance against external mechanical influences such as scratches and folding. Moreover, different image compositions, e.g. different density and/or colour composition, may lead to topographic differences on the image-receiving member which reinforce some of the above mentioned disadvantages and reveal additional disadvantages, such as gloss differences between different image parts.
U.S. Pat. No. 6,250,733 discloses a halftoning method employing a single screen for all colours wherein at low image density levels, pixels are rendered by positioning image dots of the respective colours contiguous to each other instead of superimposed. This image dot-off-dot approach is advantageous with respect to graininess. U.S. Pat. No. 6,250,733 further discloses that when the sum of the image density levels of the pixels of an image part exceeds the threshold corresponding to an ink area coverage of 100%, the remaining image density levels are rendered by superimposing image dots in a second layer of ink employing the same screen. However, when this second layer of ink is applied, care is taken not to impose an ink dot of the second layer on an ink dot of the first layer of the same colour. In order to enable this, once it is established that an image part has a sum of image density levels exceeding the threshold of 100% ink area coverage and thus image dots of a second layer need to be formed, the image data associated with the process colours are reconfigured such as to associate the image data with virtual colours. These virtual colours are realized by a combination of two process colours. Such a combination of process colours is realized by imposing two process colours onto each other in two layers, however, using the same screen. Using the same screen for both layers may cause undesired interference patterns, e.g., patterns caused by registering errors.